The determination of the volume flow of breathing gases is of considerable relevance in the area of anesthesia devices and respirators, because, for example, the absolute quantity of an anesthetic supplied to a patient during anesthesia can be derived therefrom. The absolute quantity supplied of an anesthetic is, in turn, a critical parameter in anesthesia, which should be known as precisely as possible. Therefore, it is important, particularly in medical engineering, to determine this variable as exactly as possible.
To determine the breathing gas volume flow, hot wire anemometers are known from the state of the art, which operate in such a way that the resistance of a hot wire, which is arranged in the breathing gas flow, is determined. Here, the hot wire is made of a material that has a temperature-dependent resistance. The extent to which the hot wire is cooled by the gas flow depends on the volume flow and thus on the flow velocity, so that the resistance of the hot wire is an indication of the flow velocity. However, a drawback of this method is that the hot wire itself exhibits a flow resistance and that, furthermore, only the absolute value of the flow velocity can be determined from the resistance of the hot wire. Information about the direction of flow is not possible, however.
As an alternative to hot wire anemometry, it has become known, for example, from EP 0 347 096 A2 to use the run time of ultrasonic pulses that pass through a measuring section with an ultrasound transmitter and an ultrasound receiver, which measuring section runs in parallel to the flow duct, to measure the flow velocity of gases. Here, the run time of the ultrasonic pulse is measured both against the direction of flow and with the direction of flow, and the flow velocity and its direction are determined from the run time difference.
However, problems arise in the measurement of the run times. To determine the point in time, at which the ultrasonic pulse impacts the receiver, a level has to be set, which is used as the stop level, so that the time measurement is stopped when the level of the signal of the receiver exceeds this stop level. However, in contrast to liquids, the difficulty arises that the damping which the ultrasonic pulse experiences in the gas flow varies greatly. This in turn results in that inaccuracies occur in the determination of the point in time of the impact on the receiver, since the stop level is exceeded only relatively “late,” for example, in case of a strong damping.
To this end, EP 0 347 096 A2 suggests adding a phase jump into the signal sent by the ultrasound transmitter, which can be used as a marker and whose impact can be detected by the receiver as a stop signal. However, the drawback of this type of method is that this can be carried out only at relatively long sound paths. Moreover, the problem arises in case of strong dampings that either the amplitude, with which the ultrasound transmitter transmits the signal, or the amplification of the signal of the receiver has to be increased or adapted.
Another fundamental problem in the measurement of the run time of the ultrasonic pulse is that the measured run time between the transmitting pulse at the ultrasound transmitter sent by the control electronics and the reception of the ultrasonic pulse at the receiver includes a delay in the transmission of the pulse, i.e., a delay time between the transmitting pulse at the ultrasound transmitter and the actual transmission of the signal by the ultrasound transmitter.
If it is intended to determine this delay, the exact length of the measuring section is needed in case of known sound velocity of the gas. Furthermore, the exact measuring section length is necessary for an accurate calibration. However, the further problem arises here that, on the one hand, there are tolerances in the manufacture of the flow ducts and the measuring section length is thus only known with a certain inaccuracy. On the other hand, a cleaning of the flow duct connected with the assembly and disassembly of the ultrasound transmitter and of the ultrasound receiver may lead to a change in the measuring section length. Therefore, it is desirable for an exact measurement of the flow velocity to provide a method that makes it possible to use the exact measuring section length during operation. It is thus advantageous to determine the exact measuring section length before or during the measuring operation in a routine (initialization).
Another difficulty in the determination of the flow velocity from the measured run times is that the flow velocity can be determined from the run times basically only with the knowledge of the sound velocity. However, if the composition of the breathing gas or its temperature changes, for example, in a respirator that is used for anesthesia, the sound velocity in the breathing gas also changes.
Thus, besides the run time measurement, another measurement of the current sound velocity would have to occur for the determination of the flow velocity. To this end, it has become known from U.S. Pat. No. 5,461,931 to measure the run time of an ultrasonic pulse and thus to determine the sound velocity in an area of the sensor, in which the flow rests. However, this is very expensive.
In the exceptional case in which measuring sections have exactly the same length, the determination of the flow velocity against and with the flow from the run times becomes independent from the sound velocity. This can be done only with great difficulty in a sensor that uses two measuring sections for the two directions, which results, on the one hand, from the manufacturing tolerances as well as the assembly quality and, on the other hand, from changes in the disassembly and assembly before and after the cleaning.